This invention relates to a frequency converter for producing polyphase alternating current having an improved waveform, more particularly a frequency converter wherein a transformer is used to combine or synthesize rectangular output currents for the purpose of obtaining an output current having a waveform close to that of a sine wave.
Inverter circuits adapted to convert a DC power into an AC power are classified into a voltage type inverter and a current type inverter depending upon whether their outputs have a rectangular wave voltage or a rectangular wave current. In the former, a large capacity capacitor or a battery is connected on the DC source side so that its source impedance is small and the inverter can drive any type of AC load but in this type current control is difficult whereby there is a defect that large current flows at the time of fault, for example DC short circuiting. On the other hand, in the current type inverter since a large DC reactor is connected on the DC side so that current control can be made relatively readily and this type is suitable to use for rapidly varying AC loads.
FIG. 1 is a block diagram showing a prior art current type inverter comprising a three phase AC source 11, a rectifier 12, a DC reactor 13 connected in series with the DC circuit, an inverter 14 and a three phase AC load 15.
FIG. 2 shows one example of the prior art inverter circuit 14 comprising three parallely connected branch circuits each including serially connected controlled semiconductor elements, for example thyristors 211, 212 and diodes 211, 222; thyristors 213, 214 and diodes 223, 224; and thyristors 215, 216 and diodes 225, 226 and commutating capacitors 231 through 236 respectively connected between adjacent branch circuits. The AC output lines are connected between diodes 221 and 222, between diodes 223 and 224 and between diodes 223 and 224 respectively. The operation of this inverter circuit has been well known. Briefly stated, as shown in FIG. 3, the waveforms of the output currents I.sub.R, I.sub.S and I.sub.T of respective phases are rectangular in which the positive and negative currents flow for intervals of 120.degree.. In a three phase inverter as shown in FIG. 2, adjacent positive and negative currents have a phase difference of 60.degree.. Such current wave contains a larger amount of higher harmonics. Especially, it contains 20% and 70%, based on the fundamental wave, of the fifth and seventh higher harmonics which are of relatively lower orders. For this reason, where the AC load 15 comprises an AC motor, a rotating field due to the fifth and seventh higher harmonics would be formed thus creating torque ripples having a value 6 times of that of the fundamental wave.